Direction finder



2,1947. 1.. E. NORTON 2,425,386

DIRECTION FINDER Filed May 29, 194; 2 Sheets-Sheet 1 lll M/c'EO WW:[ea-arse P/l/RSE Dan-one ems: l 65 mw/mme J p Z'mnentor attorney Aug.12, 1947. 1.. E. NORTON 2,425,386

DIRECTION FINDER Filed May 29, 1943 2 Sheets-Sheet 2 Patented Aug. 12,1947 DIRECTION FINDER Lowell E. Norton, Princeton, N. J., assignor toRadio Corporation of America, a corporation of Delaware Application May29, 1943, Serial No. 489,034

Claims. 1

This invention relates to radio direction finders, and more particularlyto improvements in the art of determining the azimuth of arrival of aradio wave by means of a continuously revolving collector system, asdescribed in detail in copending U. S. application Ser. No. 487,678,filed on May 20, 1943, by L. E. Norton, entitled Direction finders.Revolution of the collector system pro-- duces phase modulation of thecarrier of a received signal. The absolute phase of this modulation is afunction of the azimuth of wave arrival. One method of determiningazimuth in response to the modulation introduced by antenna revolutionis described in the above mentioned application.

The principal object of the present invention is to provide an improvedmethod of and means for indicating azimuth of wave arrival in responseto phase modulation produced at the receiving antenna.

Another object is to provide an improved method of and means forindicating the elevation of wave arrival in response to phase modulationproduced at the receiving antenna.

A further object is to provide a fully automatic direction indicatingsystem of the described type.

These and other objects will become apparent to those skilled in the artupon consideration of the following description with reference to theaccompanying drawing, of which:

Fig. l is a elevational view of the structure of a direction finderaccording to the present invention,

Fig. 2 is a graph representing the angular relationships involved in theoperation of the system,

Fig. 3 is a schematic block diagram of an indicator system according tothe invention, and

Fig. 4 is a schematic circuit diagram of a phase responsive servosystem.

Referring to Fig. 1, a wave collector comprising a vertical dipoleantenna i is supported at one end of a horizontal boom 3. Any desiredtype of collector element may be substituted for the di-- pole I,although a non directive element such as a vertical dipole or ahorizontal loop is to be preferred. The boom 3 is rotatably supported atthe upper end of a tower 5 and connected to a shaft 1 extending witlnnthe tower. The necessary radio receiver equipment is supported in ahousing I l which is adjacent the antenna I. POW- er for the operationof the equipment may be provided by batteries or by means of a generatorcontained within the housing I! and coupled to a shaft l3 extendingalong the boom 3. A counter.

2 weight 9 is provided at the other end of the boom 3. The shaft 53 isconnected through a pinion l5 to a gear ll provided at the top of thetower 5. The lower end of the shaft 1 is coupled through gearing E9 to amotor 2!. The described structure is similar to that described incopending application Ser. No. 488,018, filed on May 22, 1943, by L. E.Norton. The motor 2! drives the shaft 1 at constant speed, moving theboom and hence the antenna l in a circular path. This movement producesphase modulation of signals picked up by the antenna, owing to thecyclical variation in the distance between the antenna and the radiationsource.

Referring to Fig. 3, a phase detector system, generally designated bythe reference numeral 23, of the type described in the aforementionedcopendi'ng application, is connected to the antenna I. The phasedetector 23 comprises a local radio frequency oscillator 25, a fixedintermediate frequency oscillator 21, a balanced modulator 29 connectedto the oscillators 25, and 2'! and a modulator 3| connected to theoutput circuit of the modulator 29. A radio frequency amplifier 33 isprovided in the input circuit from the antenna I and is coupled to themodulator 3|. The tuning controls of the oscillator 25 and the amplifier33 are mechanically ganged, as indicated by the dash line 35. The outputcircuit of the modulator Si is coupled to a filter 3? which is tuned topass voltages of intermediate frequency.

The oscillator 25 is arranged to be controlled over a narrow range offrequency by means of a D.-C. control voltage. An automatic frequencycontrol circuit 39 for this purpose is coupled to the the filter 31 andhence to the oscillator 25. The A.-F.-C. circuit 39 is also connectedthrough a phase shifter ll to the output circuit of the I.-F. oscillator27.

A modulator 43 is connected to the output of the amplifier 33. Amodulator 45 is connected to the output of the oscillator 25. A commonbeat oscillator 4! is connected to the modulators 43 and 45. Filters 44and Z6, tuned to pass the intermediate frequency are connected to theoutput circuits of the modulators t3 and t5, and to a phase responsiveservo system 49.

The servo system 49 is provided with an output shaft 5! and is arrangedto cause rotation of said shaft to an angular position corresponding tothe difference in phase between the two inputs from the modulators 43and 35. The shaft 5| is mechanically coupled to a voltage control device53 which may be a variable mutual inductance regulator of the variactype or any other niechemically controllable regulator known to thoseskilled in the art. A constant voltage input is supplied to the controldevice 53 from a source not shown, such as A.-C. power lines.

The output of the control device 53 is applied to a detector 55,comprising a diode rectifier or the like and a filter arranged to reject components of the power supply frequency. The output of the detector55 is applied to a phase responsive servo system 57 which is identicalin construction with the servo system 49. An alternator 59 is coupled tothe antenna shaft 1 by connection to the shaft 13, for example (Fig. 1),to provide one cycle for each revolution of the wave collector. Theoutput of the generator 59 is applied to the servo system 51. The outputshaft 6! of the servo system 51 is coupled to an indicator 63. A scale65, cali-- brated in terms of azimuth, is provided for cooperation withthe pointer 63.

The operation of the above described system is as follows:

The amplifier 33 and the oscillator 25 are tuned to the frequency of thedesired signal. The outputs of the oscillator 25 and the oscillator 21are combined in the balanced modulator 29 to provide a voltage differingin frequency from the carrier by substantially I.-F. frequency. Theoutput of the modulator 29 is combined in the modulator 3| with theoutput of the amplifier 33 to provide a signal of 1-H. frequencycarrying the phase modulation of the incoming carrier. The filter 3Tremoves all components of carrier and higher frequencies from the outputof the modulator 3|.

Initially, the output of the oscillator 25 may not be of exactly thesame frequency as the incoming carrier. In this event the output of thebalanced modulator will not differ from the carrier frequency by exactlythe frequency of the oscillator 21, and the output of the filter 31 willbe of a frequency correspondingl differing from the I-F, frequency.These two'frequencies are compared in the A.F.-C. circuit 39 to producea D.-C. voltage corresponding in magnitude and polarity to theirdifference. The D.C. output of the A.-F.-C. circuit 39 is applied to thecontrol circuit of the oscillator 25, adjusting the frequency of thelocally generated R.-F. voltage so that the two I.F. voltages are equalin frequency. At this time, the frequency of operation of the oscillator25 will be exactly equal to the carrier frequency.

The phase detector 23 provides two outputs: phase modulated R.-F.carrier voltage which is applied to the modulator 53, and unmodulatedR.-F. voltage of carrier frequency which is applied to the modulator 45.The output of the amplifier 33 may be represented as:

where r is the radius of the path swept by antenna, l/ is the elevationof wave arrival, p/271' is the frequency of revolution of the antenna,and is the azimuth of wave arrival. Referring to Fig. 2, the angularposition Pt of the antenna and the azimuth angle 0 are measured from apredetermined reference line AA, such as the local meridian. The outputof the oscillator 25 may be represented as:

e1=E1 cos wt The output of the beat oscillator 41 is:

The voltages eoE are applied to the modula- 4 tor 43 and the voltages c1and c2 are applied to the modulator 45. The output of the modulator 43includes, among other components, a voltage:

All components other than as are removed by the filter 44. The voltagesc1 and ez are applied to themodulator A5. The output from the filter 46is:

The voltages es and 6'3 are applied to the servo system 49, causing theshaft 5| to be displaced to variable angular position :21AM cos 50 cosH-0) The shaft of the voltage control device 53 is displacedcorrespondingly. The output of the voltage control device 53 is:

where mot/211' is the frequency of the input potential from the powerline to the control device 53. The regulator 53 is so connected to theshaft 5| that The voltage a; is applied to the detector 55, providing anoutput including a voltage The voltage es is applied to a servo system51 together with the output es=Es cos H-A) from the generator 59. Thegenerator 59 is so coupled to the shaft 1 that A=0, if the antennarotation angle t is measured from the line A-A in Fig. 2. The shaft 6!of the servo system 51 is rotated to an angular position correspondingto the difference in phase between the output of the generator 59 andthe output of the detector 55. The angular position of the shaft 6| towhich is attached a pointer and 360 scale is 0, the azimuth of wavearrival.

The shaft 5! of the servo device 49 swings continuously with rotation ofthe collector element. The magnitude of this swing is proportional to21r1/)\ cos 1/, and consequently the output of the detector 55 is alsoproportional to cos 1//. This fact may be utilized to determineelevation of wave arrival by calibrating the system with a locallygenerated horizontally incident signal over the range of frequenciesthrough which the direction finder is to be used. The angle =cos' edet./e det. max(4=0).

Referring to Fig. 4 the circuit employed in the phase responsive servosystems comprises a variable phase shifter 61 connected to one of theinput circuits and in push-pull to the outer control grids of a pair ofmulti-grid electron discharge tubes 69 and 1 l. Another input circuit isconnected to the inner control'grids of the tubes 69 and 1| in parallel.Series resistors 13 are included in the connections to the grids of thetubes 69 and H. The anodes of the tubes 69 and H are connected throughload resistors 15 and TI to a D.-C. source 19. A point of intermediatevoltage on the source 19 is connected to the screen grids of the tubes69 and 1 I. The load resistors 15 and T! are shunted by capacitors 8|and 83, respectively.

A reversible D.-C. motor is connected between the anodes of the tubes 69and H. The shaft of the motor 85 is connected, as indicated by the dashline 81, to the control shaft of the phase shifter-6T. The 360 phaseshifter 61 includes capacitors 89 and 91 and resistors 93 and 95connected in a bridge network, and a voltage divider comprising aresistor 91 connected to the terminals of the bridge. Two variablecontacts 99 and ID! are arranged to be moved over the resistor 9'! andare mechanically connected together and to the control shaft, asindicated by the dash line [03. The capacitors 89 and 9| and theresistors 93 and 95 are so proportioned as to provide phase shifts of+90 and 90, respectively, at the junction points I05 and I! between theelements 39, 93 and 1-H, 95 respectively.

The operation of the above described servo system is as follows:

The two input voltages are of equal frequency. Assume that input No. 1lags input No. 2 by the angle 0, and that the phase shifter 61 isadjusted to introduce a lead of any magnitude 5. The input to the outercontrol grids of the tubes 69 and 7! will then differ in phase from theinput to the inner control grids of the tubes 69 and H by angles of 3-0and 0+1r-B, respectively. The control grids are normally biased by meansof the common cathode resistor 14 to cut off the plate currents. Duringthe time that both inner and outer control grids are positive, the tubeswill conduct. Depending upon which is greater, 8-0 or 9+7r-B, one of thetubes 69 and II will conduct more of the time than the other. Thepulsating plate currents are integrated by the capacitors BI and 83.Thus the average current and hence the voltage drop across one of theresistors i and T! will be greater than the other and the resultantvoltage applied to the motor 85 will correspond in magnitude to thedifference between B-0 and 6+1r-B. The motor 85 runs in one direction orthe other to drive the phase shifter 61 to such a position thatB0=0+1rB, or B=0+1r/2, whereupon the voltages across the load resistors75 and 77 are equal and the resultant voltage applied to the motor 85 iszero. At this time, the position of the control shaft of the phaseshifter 51 corresponds to the angle 0+1r/2. As shown in Fig. 3 thepointer 63 is set at the angle of 11/2 with respect to the voltagedivider, so as to indicate on the scale 65 the difference in phasebetween the two inputs to the system.

As an alternative to having all of the direction finder equipmentlocated in the housing II and rotating with the antenna, the radiofrequency amplifier 33 may be used to modulate an ultra high frequencytransmitter. A corresponding ultra high frequency receiver Ill may thenbe provided at a constant distance from the antenna; i. e., at thecenter of rotation of the system, and the output of this receiver usedto operate the equipment described above.

I claim as my invention:

1. In a radio direction finder system providing phase modulation of thecarrier of an arriving wave, the method of determining the azimuth ofarrival of said wave, comprising the steps of locally generating amodulated voltage having a frequency equal to that of the carrier ofsaid wave, converting said phase modulated signal to an intermediatefrequency signal bearing corresponding phase modulation, converting saidlocally generated voltage to a modulated voltage of intermediatefrequency, continuously comparing said intermediate frequency voltage inphase to derive a third voltage having a frequency equal to that of saidphase modulation of said signal carrier, locally generating a fourthvoltage of constant phase having a frequency equal to said phasemodulation of said signal carrier, and comparing the phases of saidthird and fourth voltages.

2. In a radio direction finder system providing phase modulation of thecarrier of an arriving wave, the method of determining the azimuth ofarrival of said wave, including the steps of locally generating anunmodulated voltage having a frequency equal to that of said carrier,continuousiy comparing in phase said locally generated voltage and saidcarrier voltage to derive a voltage having a frequency equal to that ofsaid phase modulation, locally generating a constant phase voltage of afrequency equal to that of said phase modulation, and comparing in phasesaid locally generated phase modulation frequency voltage with saidderived phase modulation frequency voltage.

3. A radio direction finder system including a revoluble wave collector,means for revolving said collector in a closed path about apredetermined point whereby a signal derived from said collector from anarriving wave is phase modulated by a frequency equal to the frequencyof revolution of said collector and in an absolute phase bearing apredetermined relationship to the azimuth of arrival of said wave, meansfor locally generating an unmodulated voltage equal in frequency to thecarrier of said signal, means for comparing the phases of said locallygenerated voltage and said carrier to control a voltage control device,a source of constant amplitude voltage connected through said voltageregulator to a detector, means for generating a voltage having aninstantaneous amplitude which is a predetermined function of the angularposition of said wave collector with respect to a reference line, andmeans responsive to the phases of said voltage and the output of saiddetector to actuate an azimuth indicator.

4. The invention as set forth in claim 3 including means for separatelyconverting said modulated carrier and said locally generated unmodulatedvoltage of carrier frequency to voltages of equal low frequency.

5. The invention as set forth in claim 3 including means responsive tothe output of said detector to indicate the elevation of arrival of saidwave.

LOWELL E. NORTON.

